1. Field of the Invention
The present invention relates to electron guns for a color cathode ray tube, and more particularly, to an electrode of electron guns for a color cathode ray tube which constitutes a large diameter electron lens.
2. Description of the Related Art
In general, in electron guns, since spherical aberration and focusing characteristics are largely affected by a main lens, the diameter of the main lens must be as large as possible in order to attain excellent focusing characteristics.
However, in in-line electron guns, since three electron beam-passing holes are formed, in a line, in at least two electrodes forming electron lenses, and the diameter of the neck portion of a funnel in which the electron guns are installed is restricted by the design requirements of deflection and convergence yokes, etc., it is not possible to make the diameter of the electron beam-passing holes larger than the distance between the centers of the two electron beam-passing holes.
A structure of electron guns for improving spherical aberration in a conventional main lens is disclosed in U.S. Pat. No. 4,370,592, and is shown in FIG. 1. As shown in FIG. 1, burring portions 5b and 6b are formed at the inner peripheries of the exit plane 5a of a focusing electrode 5 and the entrance plane 6a of a final accelerating electrode 6, respectively, and large diameter electron beam-passing holes (hereinafter referred to as large apertures) 5H and 6H having a predetermined depth are formed in the central portions of the planes. Also, small diameter electron beam-passing holes (hereinafter referred to as small apertures) 5H' and 6H' through which R, G and B electron beams independently pass, respectively, are formed in the large apertures 5H and 6H.
When the electron beams pass through the main lens formed by the focusing electrode 5 and the last accelerating electrode 6, since the large apertures 5H and 6H are non-circular or oblong and therefore vertical and horizontal convergent components of an electron beam having passed through the central small aperture and electron beams having passed through two side small apertures are different from each other, electron beam spots on a phosphor surface one not uniform. That is, as shown in FIG. 2, two side beams RB and BB passing through the large aperture 5H or 6H of the focusing electrode 5 or the last accelerating electrode 6 are horizontally close to the burring portions 5b and 6b where a low or high voltage is distributed, and the central electron beam GB is relatively far from the burring portions 5b and 6b. Accordingly, the two side electron beams are converged to a relatively larger extent, and the central beam is converged to a smaller extent.
In addition, since the distances between the two side beams RB and BB and the burring portions 5b and 6b are different from each other depending on direction, the horizontal and vertical converging forces acting on the electron beams are different from each other. In addition, since the vertical distances between the central beam GB and the burring portions 5b and 6b are shorter than the horizontal distances between them, the central beam GB is subject to stronger vertical converging forces. Further, the central beam GB suffers diverging forces in diagonal directions of the large apertures 5H and 6H. Therefore, since the sections of the two side beams RB and BB having passed the main lens are generally triangular, and the section of the central beam GB has a radial shape, uniform sections of electron beams cannot be obtained.
In particular, since the diameter of the small apertures 5H' and 6H' is restricted by the diameter of the neck portion of a cathode ray tube, there is a limit in increasing the distance between the centers of the small apertures 5H' or 6H'. Further, since the diameter of the neck portion tends to be reduced to reduce deflection yoke power consumption, the separation between the small apertures 5H' or 6H' become smaller and there are problems in which the spherical aberration increases and focusing characteristics are deteriorate.
A electrode structure for solving the above problems is disclosed in U.S. Pat. No. 5,414,323. As shown in FIG. 3, in the electrode structure, an electrode plate 16 is installed at the center of an external electrode 11 in which a large aperture is formed, a small aperture 13 of a longitudinally elongated shape is formed at the center of the electrode plate 16, and the sides of the electrode plate are cut to have semi-elliptical shapes in order to form two side electron beam-passing holes 14 and 15.
By making the central small aperture longitudinally elongated, the astigmatism caused by the large aperture is offset. However, in the above electrode, eight-pole astigmatism of the central electron beam-passing hole and six-pole astigmatism of the two side electron beam-passing holes are not corrected easily.
Another example of a conventional large diameter electrode is disclosed in U.S. Pat. No. 4,626,738. As shown in FIG. 4, the electrode includes an external electrode 21 in which a large aperture is formed, and an internal electrode 22 which is installed in the external electrode 21 and in which polygonal small apertures 22R, 22G and 22B are formed. Here, the astigmatisms caused by the large aperture can be compensated for by the polygonal small apertures 22R, 22G and 22B, but the polygonal small apertures 22R, 22G and 22B are not easily manufactured.